Calcium sulphate hemihydrate( CaSO₄.1/2H₂O) is also called as the Plaster of Paris because of the reason that Paris, during 18th century, became the major center of this compound's production.
Plaster of Paris is a hemihydrate of Gypsum and gypsum ores were available in huge amounts around Paris.
Even now, Paris is a major site for the production of this hemihydrate of Calcium sulphate.
Hence the name Plaster of Paris.
